Readers that are awoken will expect a nil ->task indicating
that a wakeup has occurred. There is a mismatch between the
smp_mb() and its documentation, in that the serialization is
done between reading the task and the nil store. Furthermore,
in addition to having the overlapping of loads and stores to
waiter->task guaranteed to be ordered within that CPU, both
wake_up_process() originally and now wake_q_add() already
imply barriers upon successful calls, which serves the comment.
Just atomically do a xchg() and simplify the whole thing. We can
use relaxed semantics as before mentioned in addition to the
barrier provided by wake_q_add(), delaying there is no risk in
reordering with the actual wakeup.
Signed-off-by: Davidlohr Bueso
---
kernel/locking/rwsem-xadd.c | 17 +
1 file changed, 5 insertions(+), 12 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 1b8c1285a2aa..96e53cb4a4db 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -126,7 +126,6 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
enum rwsem_wake_type wake_type, struct wake_q_head *wake_q)
{
struct rwsem_waiter *waiter;
- struct task_struct *tsk;
struct list_head *next;
long oldcount, woken, loop, adjustment;
@@ -190,24 +189,18 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
next = sem->wait_list.next;
loop = woken;
do {
+ struct task_struct *tsk;
+
waiter = list_entry(next, struct rwsem_waiter, list);
next = waiter->list.next;
- tsk = waiter->task;
- /*
-* Make sure we do not wakeup the next reader before
-* setting the nil condition to grant the next reader;
-* otherwise we could miss the wakeup on the other
-* side and end up sleeping again. See the pairing
-* in rwsem_down_read_failed().
-*/
- smp_mb();
- waiter->task = NULL;
+
+ tsk = xchg_relaxed(&waiter->task, NULL);
wake_q_add(wake_q, tsk);
} while (--loop);
sem->wait_list.next = next;
next->prev = &sem->wait_list;
- out:
+out:
return sem;
}
--
2.8.1
